We investigated the effects of angiotensinogen (Ang), angiotensin I (Ang I), and angiotensin II (Ang II) on the fluidity of phosphatidylcholine vesicles. Changes in fluidity were assessed by changes in anisotopy values calculated from fluorescence polarization measurements. All three compounds produced an increase in membrane fluidity when localized inside the phosphatidylcholine vesicles. When placed outside the vesicles. Ang II increased bilayer rigidity (decreased fluidity), whereas Ang and Ang I produced no effect. These results suggest the possibility that these peptides may alter the fluidity of cell membranes by a direct action on the phospholipid bilayer, which may in turn interfere with receptor-mediated effects.